The prior art method of coating aluminum upon the interior surface of a glass funnel used in the manufacture of cathode ray tubes involves the resistance heating of a tungsten coil in which a small amount of aluminum is placed. As the tungsten heats, the aluminum melts, then vaporizes so as to deposit upon the workpiece. However, the tungsten coil reacts with the aluminum, degrading the purity of the aluminum deposit and also degrading the tungsten coils and requiring their periodic replacement at a substantial cost. In addition, this method typically results in areas of thinly deposited aluminum corresponding to the shadows of the electrodes connecting the resistive coil to a source of electronic potential.
Cathode sputtering is a method used in the application of thin films of materials upon planar substrates. This process involves the generation of positively charged ions in an evacuated chamber. The positively charged ions bombard a cathode comprising a source of depositing material with sufficient energy to physically eject material from the cathode surface. The material thus ejected from the cathode, most of which is neutrally charged, diffuses across the evacuated chamber and subsequently condenses upon the substrate to form the desired deposited layer. This process is further described in U.S. Pat. No. 2,146,025 issued to Penning on Feb. 7, 1939, U.S. Pat. No. 3,282,816 issued to Kay on Nov. 1, 1966, U.S. Pat. No. 3,616,450 issued to Clarke on Oct. 26, 1971, U.S. Pat. No. 3,711,398 issued on Jan. 16, 1973, also to Clarke, and U.S. Pat. No. 3,878,085 issued to Corbani on Apr. 15, 1975.
These patents describe a wide variety of geometries for planar magnetrons characterized by the use of a magnetic field for concentrating the electron density near the cathode, but all are restricted to configurations adapted to the deposition of material upon planar surfaces. Penning shows a solid cathode with a magnet exterior to the workpiece. This is inefficient in the use of the magnetic field, is especially inefficient when used in large structures, and is not adaptable to providing a variable magnetic field along the axis of the cathode for control of the deposition rate along the axis of the magnetron.
Kay has reversed the cathode/anode geometry to better adapt the magnetron to sputtering upon planar surfaces. Clark, Clark, and Corbani illustrate further refinements in the adaption of a magnetron to sputtering upon planar surfaces.
The prior art is insufficient for the efficient sputter depositing upon the interior surface of a CRT tube because it is not adaptable to deposition upon the interior surface of a cylindrical workpiece. Further, the prior art does not provide for an efficient use of the magnetic field in applications to larger structures and does not teach a way of controlling the uniformity of the deposition along the magnetron axis for applications to workpieces non-symmetrical along the magnetrons axis of symmetry.
It is therefore the principal object of the present invention to provide a sputtering apparatus for the efficient deposition of a material upon the interior surface of a substantially cylindrical workpiece.
It is further object of the present invention to provide a magnetron whereby electrons and charged ions are effectively confined to a volume proximate to the cathode by a magnetic field wherein the magnets producing the magnetic fields are placed in close proximity with the cathode surface, allowing smaller magnets to provide the required field in larger structures.
It if a further object of the present invention to provide a magnetron wherein the magnetic fields proximate to the cathode surface are variable so as to allow control of the deposition rate upon the surface of workpieces that are substantially non-cylindrical.
It is further object of the present invention to provide a device whereby the functions of cathode, material source, support and coolant retention are performed by a single element.